There’s a bunch of potential reasons why your mayo isn’t cooperating…but…
Your egg is probably colder than your oil!
Eggs are really well insulated, and if you’re like me, you tend to take them out of the fridge right before you need them. Emulsions are very sensitive to temperature differences, and everything needs to come to room temperature before you can emulsify properly. It took me a few occasional mayo failures, until I pinned down the problem when I checked the yoke temperature with an IR thermometer. Sure enough, it was 20 degrees colder than everything else. Including the surrounding egg white.
But, don’t fret! You can save it.
Restart your mayo recipe over, what you’re going to do is make a new, smaller batch of mayo. Once you get the emulsion going instead of adding more oil, you’re going to use your failed batch. But make sure you get everything to the same temperature first! Don’t make the same mistake twice, make sure both the old mayo and new mayo are both room temperature.
I hope that helps someone! I’m so happy I lazed my egg yolk today before I made another cup of white oil.